Inductors, components that store energy in a magnetic field created by electrical current, are used extensively in signal processing applications. Inductors are typically formed with conducting wire looped a predetermined number of iterations. The inductance value of each turn of a coil, and thus the total inductance value of an inductor, depends on the coil diameter, the number of turns, and other factors.
An inductor is often coupled with a capacitor, a related component that stores energy in an electric field created by a potential difference between a pair of conductors, to form an inductor-capacitor tank (“LC tank”) circuit. An LC tank circuit is configured to operate at a known resonant frequency, allowing a communications device incorporating such a circuit to generate a signal at that specific frequency or filter out a signal corresponding to that frequency. The performance of an LC tank circuit is measured by its Q factor, a dimensionless parameter that represents the tendency of a circuit to lose energy. A circuit with a higher Q factor has a lower rate of energy loss and is thus able to maintain higher signal selectivity.
It is often desirable for inductors to be tunable for use in circuits that must be operable at a range of frequencies. One type of tunable inductor may be configured by coupling multiple sets of coils and by switching between them using, in one example, field effect transistor (FET) switches. However, this adds considerable bulk to a circuit incorporating such an inductor. Furthermore, each additional component carries an inductance and capacitance value even when unused, and the numerous additional coils and their accompanying switches would increase the mutual inductance and parasitic capacitance—and thereby degrade the Q factor of the circuit—to intolerable levels.
Hence, it is desirable to have a technique for configuring a tunable inductor that does not add significantly more bulk, capacitance, or mutual inductance to the circuit incorporating it. As will be demonstrated, the present invention provides such a technique in an elegant manner.